There exist numerous apparatuses suitable for implementing this operation (resection of the cornea). The technique of sliver resection began to be implemented more than 50 years ago and apparatuses have been under development ever since. This development has been directed essentially at two points: the means for cutting the corneal disk and the means for advancing the cornea-cutting means.
Two main types of cutter means are to be found at present: blades with circular motion (such as Prof. Draeger's microkeratome, for example) and blades with alternating rectilinear motion, including those which lie in the section plane (WO 95/31143) and those, in more widespread use, which slope relative to said plane like the blade of a planing tool (e.g. U.S. Pat. No. 4,662,370).
The blade is driven in the same way, i.e. by means of a motor or turbine whose rotary outlet shaft is connected to the blade either without any modification to its motion, in which case the blade is rotary, or else with the continuous circular motion of the motor being transformed into reciprocating linear motion by a peg secured to the shaft engaging in a groove secured to the blade which then oscillates in a direction perpendicular to the direction of its advance.
The means for moving the cutter blade through the cornea to be resected have in common a ring for fixing a, base of the apparatus on the eye, which ring carries guide means that co-operate with a blade-carrier head within which the motion of the blade is sustained. These guide means are of two types, either slideways (generally rectilinear slideways) in which the blade-carrier head is slidably mounted (see document U.S. Pat. No. 4,662,370), or else a pivot about which the blade-carrier head is pivotally mounted (see document U.S. Pat. No. 5,586,980).
The movement of the blade-carrier head along the fixed guide means is driven by the hand of the surgeon. It is limited by an abutment when cutting of the cornea is to be limited to forming a flap that remains connected to the cornea. Proposals have recently been made to motorize this movement by causing rotating means carried by the blade-carrier head to co-operate with fixed means carried by the fixing ring so that the rotating means can roll without sliding along the fixed means (in this context, see the following documents: U.S. Pat. Re 35,421, U.S. Pat. No. 5,624,456, U.S. Pat. No. 5,980,543).
Present devices with motorized advance are devices which the surgeon finds complicated to use. It is not easy to adjust the beginning and the end of the cutting trajectory of the head, and there is no way to act on the travel speed of the blade-carrier head since there is only one motor for driving both the reciprocating motion of the blade and the gearing for advancing the keratome relative to the ring with a speed reduction that locks the frequency of oscillation of the blade and its speed of advance in a ratio that is defined by construction. Finally, those keratomes are not suitable for use by surgeons who prefer to apply manual control over the advance of the keratome across the ring.